Various optical diagnostics are being developed to meet new challenges in semiconductor epitaxy. New data-analysis algorithms allow near-surface alloy compositions and interface properties to be obtained by kinetic ellipsometry (KE) without any knowledge of the underlying sample structure. New information about growth is being obtained by surface-oriented probes such as laser light scattering (LLS), surface photoabsorption (SPA), anrd reflectance-difference (-anisotropy) spectroscopy (RDS/RAS). Examples are provided and likely directions of further progress discussed.

We briefly review the optics of ellipsometry, followed by discussions of a series of example applications of the technique including single films on a substrate; multilayer stacks common to silicon integrated circuit fabrication; flat panel display materials, and in situ semiconductor growth and deposition control.

Brewster Angle Reflectance Differential Spectroscopy (BARDS) has been proposed as an optical method for real-time characterization of the growth of thin films. BARDS is based on changes in the reflectivity, Rp, of parallel (p)-polarized light incident at, or near, the Brewster angle of the substrate material. Changes in R are sufficiently large to monitor layer growth, and to determine the thickness and the optical constants of the deposited film. In this paper we extend the method to multilayer film deposition. The derivative properties of R are correlated with differences in the optical constants of the two materials, and with the sharpness of their interface. We present spectra for SiO2/Si3N4/SiO2/Si, demonstrating some of these aspects of this new and effective approach to in-situ monitoring.

This paper describes an expansion of the capabilities of real time spectroscopic ellipsometry (SE) that results from simultaneous measurement of a reflectance spectrum RA along with the two spectra in the ellipsometric angles (Ψ,δ). RA provides information complementary to (Ψ,δ), but until now has not been exploited in real time spectroscopic applications. Earlier, we have developed a novel rotating-polarizer multichannel ellipsometer for real time studies of thin film growth that utilizes a photodiode array (PDA) detector for highspeed acquisition of (Ψ,δ) spectra. Employing this instrument with a 12.5 Hz polarizer rotation rate, three-parameter (3-p) data sets {Ψ(hv,t), δ(hv,t), RA(hv,t); 1.4≤hv≤4.5 eV} can now be obtained with a time resolution as short as 40 ms during film growth. A resilient new analysis approach based on mathematical inversion and least-squares fitting is described to interpret the 3-p data set. The 3-p approach has been successful in characterizing plasmaenhanced chemical vapor deposition (PECVD) of amorphous semiconductors, and provides the film dielectric functions and thicknesses independently at each time during film growth. RA(hV) exhibits deviations from the spectrum predicted in successful modeling of (Ψ,δ) spectra alone. These deviations result from diffuse light scattering by particulates in the plasma. We show how additional information can be extracted from the spectral dependence of the scattering loss.

Phase modulated Spectroscopic Ellipsometry (SE), in the spectral range from 1.5eV to 4.6eV, was employed to characterise thin film polysilicon (poly-Si) deposited by Low Pressure Chemical Vapour Deposition (LPCVD) on SiO2/Si(100) substrates as a function of process parameters. The LPCVD deposition temperature was varied from 550°C to 620°C for silane pressures ranging from 100mTorr to 230mTorr. A variation in poly-Si microstructure was observed as a function of film depth. The influence of deposition conditions on poly-Si surface morphology was quantified using both atomic force microscopy (AFM) and SE. An increase in the measured Raman TO phonon amplitude was observed for the 620°C sample set as a function of increasing LPCVD process pressure.

Spectroscopic ellipsometry has been used to characterize oxide/poly-Si/oxide with thin nitride/oxide layer. Films were deposited on Si substrate using low-pressure chemical vapor deposition (LPCVD) techniques. The measurements were taken at angles of incidence of 65 and 70 degrees in the wavelength range from 300 to 800 nm. The analysis of the data using effective medium and two-dimensional Lorentz oscillator approximations identified complete recrystallization of the poly-Si after annealing and and its transformation to µ c-Si. Three wafers taken at the sequential stages of the manufacturing process were studied. Although parameters of the thin nitride/oxide layers are strongly correlated, reasonable estimates of the thicknesses were found. The resuilts were consistent with the measured Auger electron spectroscopy (AES) profiles.

In-situ dual-wavelength ellipsometry and laser light scattering have been used to monitor growth of Si/Si1−x,Gex heterojunction bipolar transistor and multi-quantum well (MQW) structures. The growth rate of B-doped Si0 8Ge0.2 has been shown to be linear, but that of As-doped Si is non-linear, decreasing with time. Refractive index data have been obtained at the growth temperature for x = 0.15, 0.20, 0.25. Interface regions ∼ 6-20Å thickness have been detected at hetero-interfaces and during interrupted alloy growth. Period-to-period repeatability of MQW structures has been shown to be ±lML.

Silicon surfaces cleaned using standard wet chemical processes used in silicon device fabrication have been characterised by two optical techniques, spectroscopic ellipsometry (SE) and reflectance anisotropy spectrometry (RAS). Standard peroxide-based Si(100) wafer cleans were studied by SE. It is demonstrated that the residual film on the Si surface is critically dependent on both the sequence and type of cleaning steps performed. The sensitivity of RAS performed on-line under an air ambient to HF etched Si(110) and 4° off axis Si(100) surfaces is evaluated in comparison with kinetic SE data.

In the fabrication of mercuric iodide room temperature radiation detectors, as in any semiconductor process, the quality of the final device can be very sensitive to the details of the processing steps. Each processing step can either reduce the intrinsic defects and those extrinsic defects introduced by earlier steps, or it can introduce new defects. In mercuric iodide these defects can act as trapping and recombination centers, thereby degrading immediate device performance or leading to long-term reliability problems. With careful study and monitoring of each step, the process can be modified to improve the end product. In this work we used several techniques to study processing steps and their effects. Photoluminescence spectroscopy and photoionization revealed defects introduced during processing. One critical step is the formation of electrical contacts, as both the material choice and deposition method have an impact. Four point probe sheet resistance methods were used to characterize the loss of material from the contact as it reacted with or moved into the bulk semiconductor. Ellipsometry was used to characterize the intrinsic optical functions of the material, and to study the effects of surface aging on these functions. Results from this work provide suggestions for the modification and monitoring of the detector fabrication process.

Ion implantation, diffusion, epitaxy, oxidation and ion exchange are common processing steps which alter the refractive index of dielectric media. These changes can be probed non-destructively by optical methods such as infrared and ultraviolet-visible spectroscopy, and related to material structure. An overview is given of the bilinear transform of reflectance and its use in conjunction with Fourier spectral analysis for thickness and refractive index estimation. Closed-form solutions are presently available for the bilinear transformed reflectance of heteroepitaxial multilayer structures, and for materials containing graded refractive index profiles. Salient features such as positions of interfaces and refractive index steps in multilayer media; depth and width of buried inhomogeneous layers; and width of a transition region between layers of constant refractive index can be determined directly. Under certain restrictive assumptions the bilinear transformed reflectance is shown to be invertible, which allows one to determine the complete refractive index profile. Examples are presented which illustrate the determination of thickness and refractive index of individual layers in multilayered structures. Specific examples include silicon-on-insulator, and Il-V heteroepitaxial structures such as InP/InGaAs/lnP. Analysis of the reflectance of medium to high energy implanted Si or GaAs allows determination of mean damage depth and standard deviation. The invertibility of the bilinear transformed reflectance is illustrated by estimating the refractive index profile of nitrogen implanted silicon.

Semi-insulating <100> GaAs was implanted with 170 ke V H+ and P+ ions at room temperature using a PH3 source. Fourier analysis of the bilinear transformed optical reflectance data in the infrared region of the spectrum indicated the presence of two damaged layers at different depths from the surface: (i) a deep inhomogeneous layer of low damage produced by the protons and (ii) a thin amorphized surface layer which was produced by phosphorus ions. The position and refractive index at the peak of the assumed gaussian damage profile caused by the protons, as well as the standard deviation of the profile, were estimated rapidly from the processed data. The thickness and refractive index of the surface layer were also estimated from this analysis. The presence of the amorphized surface layer was evident from the reflectance in the ultraviolet where shift and broadening of the reflectance peaks associated with the Van Hove singularities, were observed.

Three BPSG calibration sets were analyzed using infrared external reflection-absorption spectroscopy: 1) 21 films deposited on undoped silicon coated with 0.1 pm of silicon dioxide, 2) 21 films deposited on undoped silicon, and 3) 9 films deposited on microelectronics product wafers. A multivariate partial least squares analysis of the spectral data for the first large data set showed that boron content, phosphorus content, and film thickness can be quantified with precisions of 0.10 wt%, 0.12 wt%, and 30 Å, respectively. The second large data set yielded similar results. The precisions obtained for the nine product wafer samples were 0.13 wt% B, 0.09 wt% P, and for film thickness 103 Å.

Near normal-incidence spectral reflectance has been used to monitor the growth of AlAs, GaAs, and AlGaAs thin films by MOCVD in real time. The method has been used in a research rotating disk CVD reactor and in a modified commercial horizontal channel reactor. Reflectance is simple, robust, and is insensitive to perturbations such as mechanical strains, wafer rotation, and imperfect windows commonly encountered in an actual MOCVD system. Data may be collected over a wide spectral range, 390 to 950 nm, in less than one second using a multichannel detector and broad-band light source. This wide bandwidth provides detailed compositional discrimination and greater thickness sensitivity than single-wavelength measurements. The technique may be used to extract wavelength-dependent optical constant data under growth temperature conditions. Examples from the growth of multi-layered structures used in the fabrication of Vertical Cavity Surface Emitting Lasers is presented. The method shows promise as a valuable tool in process modeling, optimization, and control.

Pyrometry Interferometry (PI) is a powerful technique for in-situ sensing of the wafer temperature and growth rate. Evaluation of the two parameters would allow exact process control required for sophisticated device fabrication and material processing. The PI technique analyzes the interference patterns of the thermal radiation from the growing layer with a changing thickness d at growth temperature T. Since it is non-contact, applicable to all semiconductor materials and insensitive to wafer motion, the method is an ideal candidate for real time process control. We use a reflection assisted method to aid real time computation of these parameters. One could select the wavlength of interest to optimize the temperature and layer thickness resolution. We present data on MBE grown quarter wavelength stacks of GaAs and AlAs, and silicon oxidation to show P1 is extremely useful for growth of surface emitting laser and for silicon processing.

In this paper the application of microscope-spectrophotometry to the nondestructive characterization of a variety of multi-layer GaAs/A1GaAs structures, is described. Spectral reflectance results are used to indirectly determine variations in aluminium content, and the interdependency of aluminium content with layer thicknesses. The penetration depth of light from the visible spectrum is assessed from the correlation between spectral reflectance measurements and fitted optical models. Finally, a series of single quantum wells are investigated, and it is concluded that a significant improvement in the characterization of these materials will be achieved with an extension of the spectral measurement range into the ultra violet.

In-situ measurements of diffuse light scattering at λ = 457 nm are reported from the surface of GaAs films during growth by molecular beam epitaxy. Three different scattering angles are measured simultaneously corresponding to spatial frequencies in the surface roughness of q = 0.9, 12, and 17 μm−1. During growth the initial surface roughness caused by the oxide desorption decreases at high spatial frequencies and increases at low spatial frequency. The low spatial frequency roughness corresponding to scattering vectors parallel to [110] increases more rapidly during growth than for scattering parallel to [111].

We report the real-time in situ observation of heterointerface dislocation formation during the growth of lattice-mismatched widegap II-VI/GaAs heterostructures. Such observations were made by employing a near-normal incidence HeNe laser probe during epitaxial growth which generated both a laser reflection interferometry (LRI) signal as well as an elastically scattered laser light (ELLS) signal. We believe that the scattered light signal is generated at the II-VI/GaAs heterointerface based on the observation of a π phase difference between the LRI and the ELLS signals which were monitored simultaneously. We suggest, therefore, that the observed ELLS signal is a consequence of dislocation formation at the heterointerface which occurs due to plastic deformation in lattice-mismatched systems.

The premixability of reagents used in chemical vapor deposition reactors is important to insure that gas feed lines and nozzles do not become clogged with particulates during operation. Even if reactants are to be kept separate until introduced into a reaction chamber, it is desirable to limit the number of particles formed. A reactor which utilizes laser light scattering to monitor particulate formation when gaseous reagents are mixed is described. The reaction of tin (IV) chloride with water is commonly used to produce tin oxide films by chemical vapor deposition. It was found by the light scattering experiment that at temperatures above about 110°C the number of particulates formed is greatly reduced. Therefore, it would be most desirable that these reagents be mixed above this temperature when depositing tin oxide from this reaction. The reaction of titanium tetrachloride with various amine was also investigated by this method. This reaction has been demonstrated to produce titanium nitride above 450°C. For each case, it was observed that there was a temperature above which the number of particulates was significantly reduced. This temperature was always below the optimal temperature for producing titanium nitride films.

This paper will review the use of contactless electromodulation methods, such as photoreflectance (PR) and contactless electroreflectance (CER), to characterize the electronic properties of compound semiconductor surfaces exposed to different growth and post-growth conditions. Also the characterization of properties critical to device performance can be evaluated. For example, using PR and CER it has been found that there is a lower density of surface hole traps than electron traps in certain as-grown MBE (001) GaAs samples and that this condition persists even after air exposure. This behaviour is in contrast to other samples, including both bulk and MBE grown (001) surfaces in which the Fermi level is pinned mid-gap for both n- and p-type structures. We also have observed that Ar+ bombardment under UHV conditions results in Fermi level pinning close to the conduction band edge and that thermal annealing restores mid-gap pinning. Finally, using PR we are able to characterize the electric fields and associated doping levels in the emitter and collector regions of heterojunction bipolar transistor structures (fabricated from III-V materials), thus demonstrating the ability to perform inprocess evaluation of important device parameters.

Photoreflectance has been used to characterize the etch-induced damage in GaAs processed in an Ar/Cl2 plasma generated by an electron-cyclotron resonance (ECR) source. We show that the damage is localized to the surface and that it is most influenced by the RF power, with little effect from the microwave power. The Fermi-level is observed to be unchanged in n-GaAs and remains near midgap, while for p-GaAs, the Fermi level shifts from near the valence band to midgap. Etch-induced anisite defects are proposed as a possible source of the damage.